Correlating the 410 km discontinuity low-velocity layer occurrence with velocity tomograms using Pds imaging with high-fold arrays

使用高倍阵列 Pds 成像将 410 km 不连续低速层出现与速度断层图关联起来

• 批准号: 1045810
• 负责人: Kenneth Dueker
• 金额: $ 11.24万
• 依托单位: University of Wyoming
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-02-15 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1045810&HistoricalAwards=false
• 关键词: Correlating; 410; km; discontinuity; low

Two new seismic observations of the upper mantle beneath the western US are: 1) a low velocity layer atop the 410 km discontinuity is often found from stacking receiver function data from high fold PASSCAL arrays; 2) about eight low velocity anomalies are observed that extend from the uppermost mantle to near the 410 km discontinuity (Schmandt and Humphreys, 2010). The best interpretation of the low velocity layer atop the 410 km discontinuity is that it manifests the presence of a partially molten layer created by the water-filter model (Bercovici and Karato, 2003). We suggest that the numerous low velocity anomalies above the 410 km discontinuity may be the manifestation of buoyant melt diapirs that nucleate from the top of a buoyant 410 km discontinuity melt layer. This scenario of upwelling 410 melt layer diapirs has been studied and shown to be plausible (Leahy and Bercovici, 2008). Our prior work imaging the 410 km discontinuity demonstrates that high fold PASSCAL arrays with 1 year of recording and a station spacing of 30 km are required to reliably image a 410 low velocity layer. We propose to image the 410 km discontinuity using the following PASSCAL/Flexible arrays: the Sierra Nevada Earthscope Array, the Flex-Array Mendocino Experiment, the Colorado Rockies Experiment and Seismic Transects array, and the High Lava Plains Array. The 410 km discontinuity will be modeled using a grid-search algorithm and a simple velocity model parameterization; model uncertainties will be assessed by calculating the 1- and 2-dimensional model parameter marginal probability density functions. Our hypothesis - that the low velocity anomalies above the 410 km discontinuity manifest upwelling diapirs from a 410 melt layer - will be tested by correlating the 410 melt layer occurrence/thickness with P/S body wave tomogram(s). We will use the tomograms of Schmandt and Humphreys (in review) as this model has travel-time data from all the PASSCAL arrays we intend to analyze, in addition to the Transportable Array data.The western United States is a region currently experiencing significant volcanic activity, uplifting mountain ranges, and is actively being extended (e.g., to make the Basin and Range). Some portion of this tectonic activity is mostly likely driven by deep mantle processes such as upwelling mantle plumes and subducting oceanic lithosphere. This proposal intends to test a new hypothesis for how deep mantle process maybe affecting the tectonic activity of the western United States: specifically, that some of this tectonic activity maybe driven by upwelling diapirs that are shed from a partially molten melt layer that resides atop the 410 km depth mantle velocity discontinuity. This velocity discontinuity manifest the solid-state phase transition of the olivine component of the mantle, and due to differences in the olivine component water solubility above and below this phase transition, a partially molten melt layer (few percent melt porosity) maybe produced by an upward flux of sufficiently hydrate mantle from below the 410 km discontinuity. To test this hypothesis, the spatially distribution of the 410 km melt layer will be mapped using seismic data from high density PASSCAL and Flexible array seismic deployments in the western United States. Comparison of this new 410 melt layer map with new tomographic images of the velocity structure beneath this region will quantify the correlation between the numerous upper mantle low velocity ?pipes? and the 410 km melt layer occurence. The societal benefit to discovering whether the upper mantle low velocity anomalies manifest 410 melt diapirs would be an improved understanding of the origin of volcanic activity in the western US.

美国西部下方上地幔的两个新地震观测结果是：1）通过叠加来自高倍 PASSCAL 阵列的接收函数数据，经常可以发现 410 公里不连续面顶部的低速层； 2) 观测到约 8 个低速异常，从最上地幔延伸至 410 km 间断面附近 (Schmandt 和 Humphreys，2010)。 对 410 公里间断面顶部低速层的最佳解释是，它表明存在由水过滤模型创建的部分熔融层（Bercovici 和 Karato，2003 年）。我们认为，410 公里间断面上方的众多低速异常可能是浮力熔体底辟的表现，该底辟从浮力 410 公里间断面熔体层顶部成核。 这种 410 熔体层底辟上升的情景已经被研究过并被证明是合理的（Leahy 和 Bercovici，2008 年）。 我们之前对 410 公里不连续性进行成像的工作表明，需要具有 1 年记录和 30 公里站间距的高倍 PASSCAL 阵列才能对 410 低速层进行可靠成像。我们建议使用以下 PASSCAL/Flexible 阵列对 410 公里不连续性进行成像：内华达山脉 Earthscope 阵列、Flex 阵列门多西诺实验、科罗拉多洛矶实验和地震横断面阵列以及高熔岩平原阵列。 410公里的不连续性将使用网格搜索算法和简单的速度模型参数化进行建模；模型不确定性将通过计算一维和二维模型参数边际概率密度函数来评估。 我们的假设 - 410 公里间断面上方的低速异常表现为来自 410 融化层的上升底辟 - 将通过将 410 融化层出现/厚度与 P/S 体波断层扫描相关联来进行测试。我们将使用 Schmandt 和 Humphreys 的断层图（正在审查中），因为除了可移动阵列数据之外，该模型还具有来自我们打算分析的所有 PASSCAL 阵列的旅行时间数据。美国西部是一个目前正在经历严重火山活动的地区活动，抬升山脉，并正在积极扩展（例如，形成盆地和山脉）。 这种构造活动的某些部分很可能是由深部地幔过程驱动的，例如上涌的地幔柱和俯冲的海洋岩石圈。该提案旨在测试一个新的假设，即地幔深处的过程可能如何影响美国西部的构造活动：具体来说，某些构造活动可能是由上升的底辟驱动的，这些底辟是从位于地幔顶部的部分熔融的熔体层脱落的。 410公里深度地幔速度不连续性。这种速度不连续性表明了地幔橄榄石组分的固态相变，并且由于在该相变之上和之下橄榄石组分水溶性的差异，部分熔融的熔体层（百分之几的熔体孔隙率）可能由充分水合物地幔从 410 公里不连续面下方向上通量。为了检验这一假设，将使用美国西部高密度 PASSCAL 和灵活阵列地震部署的地震数据来绘制 410 公里融化层的空间分布图。将这张新的 410 熔体层图与该区域下方速度结构的新断层扫描图像进行比较，将量化众多上地幔低速“管道”之间的相关性。以及410公里熔体层的出现。发现上地幔低速异常是否表现为 410 融化底辟的社会效益将有助于加深对美国西部火山活动起源的了解。